A new study has cast doubt on the view that variations in the density of some of the deepest currents of the subpolar North Atlantic Ocean are caused by winter surface conditions and represent changes in the strength of the meridional overturning circulation (MOC).
The meridional overturning circulation is characterized by a northward flow of warm, salty water in the upper layers of the Atlantic, and a southward flow of colder, deep waters.
The research, published recently in Nature Communications, is the result of the international effort of 15 research institutes.
The project was led by Dr. Feili Li and Prof. Susan Lozier from the Georgia Institute of Technology, in partnership with Dr. Brad DeYoung, Department of Physics and Physical Oceanography at Memorial University.
Observations made from 2014 to 2018 in the subpolar North Atlantic reveal no imprint of strong winter cooling at the surface of the ocean on the density of the deepest boundary currents found in the western regions of ocean basins.
The authors also found no discernible relationship between changes in those deep western boundary currents and variations in the strength of the MOC.
“Changes in the strength of the MOC directly affect sea level, climate and weather for Europe, North America and parts of the African continent.”
Knowledge of the physical processes that govern changes in the MOC are “essential” for accurate climate projections, says Dr. de Young.
“The MOC brings vast amounts of heat and salt into the Northern Atlantic via the Gulf Stream and North Atlantic Current,” he said.
“Changes in the strength of the MOC directly affect sea level, climate and weather for Europe, North America and parts of the African continent. Climate projections all predict a slowing of the MOC as a result of greenhouse gas emissions, with potentially damaging impact on coastal communities and land.”
Model relationships underpin evidence
Previous model analysis led scientists to think that changes in the strength of the MOC are associated with changes in the density of the deep western boundary currents that make up the majority of the southward return flow of the MOC loop.
In models, the currents’ density can be strongly affected by a winter process called deep convection or deep-water formation.
The process occurs when cold winds cool the surface water, causing it to become dense and sink to depths of more than two kilometres.
The relationship in models between convection, changes in deep western boundary currents and the strength of the MOC also underpins evidence from palaeoclimate proxies for periods of reduced MOC and low European temperatures.
International community of oceanographers
In 2014 a huge array of scientific instruments was placed in the subpolar North Atlantic (OSNAP) to observe these processes in real life.
The surprising new results will stimulate a reconsideration of the notion that deep western boundary changes represent overturning characteristics, with implications for future climate projections as well as the interpretation of past climate change.
Prof. Susan Lozier is overall lead of the international OSNAP program.
She says it is “gratifying” to see what an international community of oceanographers can achieve with a concentrated effort of collaboration and determination.
“Programs such as OSNAP and RAPID are blueprints for how oceanographers across the globe can collectively study the ocean’s role in climate change in the years and decades ahead,” she said.
“These new observations of the deep ocean will lead to fundamental shifts in our understanding of how these major ocean currents operate and their role in the climate system,” said Dr. de Young.
“We will need that understanding as we chart our way forward in the face of global climate change.”